1. Field of the Invention
This invention relates generally to offshore floating vessels and offshore clear water vessels used for exploration and production of offshore oil and gas, and more particularly to an offshore floating production, storage, and off-loading vessel having a non ship-shaped hull of generally cylindrical or polygonal configuration surrounding a central double tapered conical moon pool that provides added virtual mass, increases the natural period of roll and heave modes, and reduces dynamic amplification and resonance, and contains ballast and storage compartments.
2. Background Art
The development of oil and gas fields in seas of ice-covered water, such as the Piltun-Astokhskoye field located offshore of Sakhalin Island, Russia, in the Sea of Okhotsk, present enormous design load challenges for engineers of semi-submersible vessels, and floating production, storage and off-loading (FPSO) vessels. The Sea of Okhotsk is subject to dangerous storm winds, severe waves, icing of vessels, intense snowfalls and poor visibility. The top surface of the sea is covered with ice sheets ranging in thickness of from about 1 m to 2 m and moving at speeds of 1-2 knots. Broken rubbles of ice (one year or multiyear) can build up to 25 m deep. This ice covered water environment typically lasts anywhere from 150 to 230 days, and during the ice-free period or “clear water field” days wave heights range between 1-3 m, but can reach as high as 19 m during 100-year storm conditions. These areas are also subject to frequent severe seismic activity. The water depth ranges from 40 m to 300 m.
A few arctic mobile offshore drilling units have been constructed to operate primarily in water depths from about 12 m-50 m. Sakhalin Energy Investment Company has modified and refurbished an Arctic Class Drilling Vessel, known as the Molikpaq, a single anchor leg (bottom founded steel caisson) which is an ice-resistant structure, originally built to explore for oil in the Canadian Beaufort Sea. This vessel is mobile but a bottom founded steel caisson structure with hollow central core filled with sand to provide resistance to the environmental loadings. The Molikpaq has no storage options and has been modified by adding a steel pontoon base and is installed bottom fixed in 30 m water at Piltun-Astokhskoye Field, 16 km offshore of Sakhalin Island's Northeast shore in the sea of Okhotsk. An independent Floating Storage and Offloading facility (FSO) is used in conjunction with this bottom mounted gravity fixed production platform.
Other types of platforms that are used in ice-covered waters include gravel or ice islands, fixed platforms and conventional floating platforms. Gravel or ice islands are limited to water depths up to 10 m.
Jacket type fixed platforms are incapable of withstanding the large lateral forces generated by large ice fields and ice floes. In general, water depths over 60 m could be declared deep in the Arctic zone and floating vessels are inevitable in the design. Single and multiyear pressure ridges, like 20 m-30 m drafts are strong enough to destroy the fixed arctic platforms.
There are several patents directed toward arctic platforms and vessels.
Bennett, U.S. Pat. No. 3,696,624 discloses counter-rotating bucket wheels mounted on offshore platforms or ship prows for cutting ice sheets found in frigid waters. The bucket wheels rotate in a generally horizontal plane and are paired in opposite directions so that a torque is not placed on the structure or ship. Multiple sets of bucket wheels can be used to cut a thick section of ice and/or the bucket wheels can be inclined or arranged to oscillate up and down to cut a larger vertical section. This apparatus provides an extensive and expensive mechanically powered way of managing ice for the large season of ice-covered water period in the arctic zone.
Stone, U.S. Pat. No. 3,807,179 discloses a hydraulically operated deicing system of apparatus for protecting columns of offshore structures from dynamic forces of ice in which a plurality of upwardly movable ice-lifting elements are supported around the column and means are provided for moving the elements upwardly against the ice to break large blocks of ice from the icepack. The ice-breaking elements may be combined with inclined planes adapted to exert upward forces on the ice.
Ehrlich, U.S. Pat. No. 4,103,504 discloses a semi-rigid interface between a moving ice field and a stationary offshore platform employing a plurality of cables which extend from points located around the periphery of the platform above the ice-covered water to corresponding points on the submerged portion of the structure, forming a protective shield of evenly spaced cables around the structure. The cables may then be caused to vibrate at predetermined frequencies, thereby reducing the frictional forces of the ice against the structure and additionally including a self-destructive natural frequency in the surrounding ice field. A compressible bladder or filler is used between the cables and the structure to prevent ice buildup behind the cables. This method of ice resistance is inefficient and requires maintenances of the cables. Moreover, ice forces typically are not uniform all around and are primarily in the direction of the ice flow movements. Thus, a uniform lifting of the hull due to the ice contact load to the hull is not possible. Hence, the mooring tension on the cables is different among the mooring lines. Additionally, a massive structure is required to resist large ice.
Gerwick, Jr. et al, U.S. Pat. No. 4,433,941 discloses a floating hull structure having ice-breaking capabilities which is moored by a plurality of flexible mooring lines that extend vertically from a moonpool in the hull to the marine bottom directly under the hull. The mooring lines are tensioned by tensioning means within the moonpool to draw the hull downward to a position below its normal buoyant position thereby substantially eliminating vertical heaving of the hull. When an ice mass contacts the hull, tension on the mooring lines is relaxed to allow the hull to rock upward against the ice thereby generating the forces necessary for the ice-breaking operation.
Oshima et al, U.S. Pat. No. 4,457,250 discloses a floating-type offshore structure having a main body with a lower hull and plurality of struts supporting a platform above the sea level and which is moored through mooring lines at an offshore location. The structure is adapted for use under both of an ice-covered and an iceless conditions of the sea by adjusting the amount of ballast water contained in a ballast tank or tanks formed in the lower hull and/or the struts and adapted for causing ice floes to undergo downward flexural failure on account of bending stresses when they move into the sea water along the ice contacting face of the strut which is inclined inwardly and downwardly. The contact area of the struts is limited and, thus, the efficient of the ice breaking is limited. There is also no large storage facility feasible with this structure.
There are several patents directed toward ship-shaped and vertical cylinder shaped moored floating vessels that are used for offshore oil and Liquid Natural Gas (LNG) storage in clear water applications.
Daniell, U.S. Pat. No. 4,606,673 discloses a stabilized spar buoy for deep sea operations including an elongated submerged hull having a selected volume and a selected water plane area, mooring lines connecting the bottom portions of the hull with the sea bottom. The hull has oil storage chambers and variable ballast chambers to establish and maintain a constant center of gravity of the spar buoy at a selected distance below the center of buoyancy. A riser system extends through a through passageway in the hull, and a riser float chamber having pitch oscillations of the same amplitude as the hull maintains tension on the riser system and minimizes pitch motions therein. The bending stresses in the riser system between the sea floor and the riser float chamber are minimized by maintaining a selected constant distance between the center of gravity and the center of buoyancy under different load conditions of the spar buoy. The variable ballast chambers in the hull extend above the oil storage chambers.
Smedal et al, U.S. Pat. No. 6,945,736 discloses a semi-submersible platform for drilling or production of hydrocarbons at sea, consisting of a semi-submersible platform body that supports drilling and/or production equipment on its upper surface. The platform body is designed as a vertical mainly flat bottomed cylinder which is provided with at least one peripheral circular cut-out in the lower section of the cylinder since the center of buoyancy for the submerged section of the platform is positioned lower than the center of gravity of the platform. This structure is similar to the spar structure of Daniell, U.S. Pat. No. 4,606,673, except there are no moving parts inside, and the diameter is larger than the draft, and the center of gravity is below the center of buoyancy. The circular cut-out which is relied upon to minimize the roll and pitch of the semi-submersible is relatively small compared to the diameter/draft dimension of the vessel, and the edges above and below the cut-out will create whirls in the water which runs therethrough. Thus, the efficiency of the small cut-out in dampening the roll and pitch motion and its strength in controlling the large vertical floating cylinder is reduced.
Haun, U.S. Pat. No. 6,761,508 discloses a floating Satellite separator platform (SSP) for offshore deepwater developments having motion characteristics with vertical axial symmetry and decoupling of hydrodynamic design features. A motion-damping skirt is provided around the base of the hull, which is configured to provide ease of installation for various umbilicals and risers. A retractable center assembly is used in a lowered position to adjust the center of gravity and metacentric height, reducing wind loads and moments on the structure, providing lateral areas for damping and volume for added mass for roll resistance. The center assembly is used to tune system response in conjunction with the hull damping skirt and fins. The center assembly also includes separators below the floating platform deck capable of being raised and lowered alone or as a unit serve to add stability to the floating structure by shifting the center of gravity downward.
The ship-shaped and vertical cylinder shaped moored floating vessels discussed above that are used for offshore oil and liquid natural gas (LNG) storage in clear water applications, including the spar-type structures, do not incorporate an ice-breaking or ice management system in the vessel design, nor any ice resistant shape to the outer structure. Thus, these types of vessels and platforms are not arctic class structures and are not particularly suited to withstand ice covered waters near the arctic zone.
The present invention is distinguished over the prior art in general, and these patents in particular by an offshore floating production, storage, and off-loading vessel having a monolithic non ship-shaped hull of generally cylindrical or polygonal configuration surrounding a central double tapered conical moon pool and contains water ballast and oil and/or liquefied gas storage compartments. The exterior side walls of the polygonal hull have flat surfaces and sharp corners to cut ice sheets, resist and break ice, and move ice pressure ridges away from the structure. An adjustable water ballast system induces heave, roll, pitch and surge motions of the vessel to dynamically position and maneuver the vessel to accomplish ice cutting, breaking and moving operations. The moon pool configuration provides added virtual mass capable of increasing the natural period of the roll and heave modes, reduces dynamic amplification and resonance due to waves and vessel motion, and facilitates maneuvering the vessel. The vessel may be moored by a disconnectable buoyant turret buoy which is received in a support frame at the bottom of the moon pool and to which flexible well risers and mooring lines are connected.